Does gated beam delivery impact delivery accuracy on an Elekta linac?

Abstract In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. Delivery accuracy was examined with an emphasis on the impact of using short gating windows (low monitor unit beam‐on segments) or long beam hold times. The performance was assessed using a 20cm by 20cm open field with the radiation delivered using a range of beam‐on and beam‐off time periods. Gated delivery measurements were also performed for two SBRT plans delivered using volumetric modulated arc therapy (VMAT). Tests included both free‐breathing based gating (covering a variety of gating windows) and simulated breath‐hold based gating. An IBA MatriXX 2D ion chamber array was used for data collection, and the gating accuracy at low MU was evaluated using gamma passing rates. For the 20 cm by 20 cm open field, the measurements generally showed close agreement between the gated and non‐gated beam deliveries. Discrepancies, however, began to appear with a 5‐to‐1 ratio of the beam‐off to beam‐on times. The discrepancies observed for these tight gating windows can be attributed to the small number of monitor units delivered during each beam‐on segment. Dose distribution analysis from the delivery of the two SBRT plans showed gamma passing rates (± 1%, 2%/1 mm) in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma passing rate of free‐breathing gating and breath‐hold gating deliveries were measured in the range of 95.7% to 100%. In conclusion, the results demonstrate that Elekta linacs can accurately deliver respiratory gated treatments for both free‐breathing and breath‐hold patients. Some caution should be exercised with the use of very tight gating windows.


| INTRODUCTION
Normal diaphragmatic excursion during uncontrolled breathing can result in significant respiratory-induced motion for tumors of the lung and liver. In radiation therapy, the impact of respiratory motion is typically accounted for by creating a target volume which fully encompasses the tumor movement. This approach, however, can result in large volumes of non-target tissue being irradiated. This can increase the toxicity of treatment and limit the dose that can be delivered to the tumor. Researchers have developed alternative techniques that account for respiratory motion to reduce the target volume. These techniques include tumor tracking and gated beam delivery. [1][2][3][4][5][6][7][8] Gated beam delivery has the advantage of being less technically complex as compared to multileaf collimator (MLC) based tracking.
The downside of a gated approach, however, is decreased treatment efficiency that results in longer treatment times. 9 In gated beam delivery, the linear accelerator beam is typically triggered on and off at either full-inspiration or end-expiration. The user determines a gating window, and radiation is only delivered during a specified phase of the breathing cycle. 10 One common approach to gated beam delivery is the use of a deep-inspiration breath hold (DIBH) technique for left-sided breast cancers with a goal of minimizing the dose to the heart and lung. 9 Gated delivery is also used in the treatment of solid lung cancers. 11,12 When commissioning a system for gated radiotherapy, it is important to characterize the startup characteristics of the accelerator. 13 This is true because gated radiotherapy introduces delivery situations not typically encountered in external beam radiotherapy.
With free-breathing gating, the radiation is delivered using a large number of segments. The use of a tight gating window combined with beam-on delays can result in a low number of monitor units (MUs) per deliverable segment. Previous studies have demonstrated the need to characterize beam stability for short irradiation times. [14][15][16] Additionally, for breath-hold-based gating, the beam is held for an extended period between each delivery segment. The impact of these prolonged beam-holds on the accuracy of the delivered radiation needs to be addressed.
Gated delivery techniques have been investigated for Varian linacs. 17,18 More recently, the gating characteristics (e.g., beam profile and beam delivery efficiency) have been evaluated for Elekta Precise and Synergy linacs. 10,19 In this study, we have focused on the beam startup characteristics for gated delivery of an Elekta linac and the overall accuracy of the delivery for a variety of gating scenarios.
Using an Elekta Synergy linac (Elekta AB, Stockholm, Sweden) in our clinic, gated beam delivery was performed using an Elekta For this study, we wanted to test whether the delivery accuracy would be compromised if the delivery utilized a tight gating window that resulted in the delivery of a low number of monitor units for each breathing cycle. We performed tests to validate the gated beam delivery accuracy while assessing a variety of gating windows.
Comparisons were made between the gated delivery and non-gated delivery (baseline). Deliveries were also performed without the use of the Elekta Response Gating kit. For these deliveries, each segment was delivered as a separate beam meaning the beam was not coming out of an active hold when it turned on. The gated technique was evaluated using two clinical plans that were delivered under free-breathing (FB) and breath-hold (BH) modes using a simulated breathing pattern.

2.A | Beam delivery characteristics
The gated beam delivery was triggered using the Elekta Response gating interface (Elekta AB, Stockholm, Sweden). The Elekta Response gating interface consists of a gating switch box that enables or disables the gated beam delivery. Gating signals were created using gating control software (Elekta AB, Stockholm, Sweden) that uses a digital signal (0 for beam-off and 1 for beam-on) to simulate free-breathing and breath-hold signals (Fig. 1). Gated beam deliveries were performed using a number of beam-on and beam-off combinations.
In Elekta's linear accelerator delivery control software, the user can set the maximum gun-hold time. If the delay time between beam segments in a delivery exceeds the specified maximum gun-hold time, the linac switches from an active mode to a standby mode. For this work, the maximum gun-hold time was set to the highest allowable value of 6.5 seconds. 10 The advantage of setting a long gunhold time is that the beam-on delays are significantly less when the beam is turned on out of an active beam-hold state. This results in a more efficient delivery. One of the goals of this work was to determine if there is any loss in dosimetric accuracy by setting the gun-hold value to the maximum allowed value. In other, words does setting up the system in a manner that maximizes delivery efficiency have negative consequences in terms of delivery accuracy? A 20 9 20 cm 2 open field (with 20 MU or 200 MU deliveries) was used to test the gating accuracy. First, the field was delivered in a normal mode (N mode). Next, the same field was delivered using the gated beam delivery mode (G mode). The gating windows were defined with beam-on times of 1, 3, and 5 seconds and beam-off times of 1, 3, and 5 seconds (Tables 1 and 3). | 91 deliver a 20 9 20 cm 2 field with 4 MU five times to achieve the same effect as delivering a single 20 MU field. As compared with the gated delivery where beam was held between each gating window, such a delivery requires the beam to switch on and off for each radiation delivery. For gating tests using actual patient treatment plans, two SBRT VMAT cases were used.

2.B | Phantom measurements
In the gated and static beam deliveries, the dose measurements were performed with an IBA MatriXX Evolution 2D ion chamber array inserted into a MultiCube phantom (IBA Dosimetry, Schwarzenbruck, Germany). The detector array has an active measurement area of 24 9 24 cm 2 and contains 1024 micro ion-chambers. The gated beam delivery using a variety of gating windows was carried out at 6 MV, 10 MV, and 18 MV (Table 1) (Table 4) and beam-on/off times of 6 and 12 seconds to simulate a breath-hold (BH) scenario (Table 4). 16

2.D | Data analysis
In this work, the results from G mode and M mode were compared to the result of N mode to assess the gating delivery accuracy. The OmniPro-I'mRT 1.5a (IBA Dosimetry, Schwarzenbruck, Germany) software was used to analyze the collected data based on gamma index evaluation and using the movie mode with a frame rate of 0.1 seconds. A dose grid was converted to spacing of 7.6 mm using linear interpolation. The passing rates using gamma index criteria of 1% and 2% with AE 1 mm distance-to-agreement (DTA) were determined for all measurements. 20 3 | RESULTS

3.A | Measurement reproducibility
The reproducibility of the measurements performed with the MatriXX array detector was determined for gating and nongating mode ( Table 1). The mean, standard deviation, and coefficient of variation were determined for three trials. Using a gamma score (1%/1 mm), the percentage coefficient of variation (CV) was less than 2% for G mode, and no statistically significant variation was observed for the open field (N mode). Using a gamma score of 2%/ 1mm, the measurement variation approached zero.

3.B | Dose distribution comparison of gated and non-gated beam delivery at 20 and 200 MU
For 6 MV, excellent agreement was observed between the G mode and the N mode. The gamma passing rates were greater than 99% for all of the gating windows and energies using 1 mm and 2% criteria. These findings were comparable to those obtained using a F I G . 1. User interface of the Response kit gating software (Elekta AB, Stockholm, Sweden) used to simulate a variety of scenarios for beamon/off to perform automatic gating beam delivery using a square wave. The figure shows the signal pattern for a beam-on of 1 second and beam-off of 5 seconds.

| DISCUSSION
We investigated the gated beam delivery accuracy when a small number of monitor units are delivered in each gating window. The findings regarding measurement reproducibility were similar to those reported by Elizabeth et al. 21 Close agreement in dose distribution comparisons were found for both gated and non-gated beam deliveries using both an open field and VMAT delivery techniques. When the gating window was reduced to 17% (1 second on, 5 seconds off), a reduced dosimetric accuracy was observed (Table 1).   T A B L E 4 Gamma score (1%/1mm; 2%/1mm) for gated beam delivery for patient A and B using selected beam-on/off time delivery. Gated beam delivery for FB mode with the beam-on/off time of (1:3), (1:5), (2:2), and (3:1) and for BH mode with the beam-on/off time of (12:6). 2. An example of time-resolved symmetry for gated beam delivery of 20 MU. (a) using beam-on/off time of (1 s:1 s), (1 s:3 s), and (1 s:5 s) and (b) static beam delivery of 20 MU using segment of 2MU 9 10, 4MU 9 5, and 10MU 9 2.